This invention relates generally to Magnetic Resonance Imaging (MRI) and more particularly the invention relates to the spectral-spatial magnetic excitation selective simultaneously in two spatial dimensions and in the spectral (i.e. chemical shift) dimension.
Magnetic resonance imaging (MRI) is a non-destructive method for the analysis of materials and represents a new approach to medical imaging. It is generally noninvasive and does not involve ionizing radiation. In very general terms, nuclear magnetic moments are excited at specific spin precession frequencies which are proportional to the local magnetic field. The radio-frequency signals resulting from the precession of these spins are received using pickup coils. By manipulating the magnetic fields, an array of signals is provided representing different regions of the volume. These are combined to produce a volumetric image of the nuclear spin density of the body.
Spectrally selective pulses are used to image selected components in an object, such as fat and water, or to image metabolites with very low density such as N-acetyl aspartate (NAA), creatine, and choline in the presence of fat and water. However, due to the small chemical shift between individual components (fat and NAA, for example), inhomogeneities in the magnetic field can cause undesired signals to be imaged while wanted signal components move out of the excitation passband. This is a particular problem in "fat saturation," where fat is selectively excited and dephased prior to excitation and imaging of water. Inhomogeneity in the static field, B.sub.0, can cause fat to fall outside the excitation passband or cause water to fall inside the passband, resulting in lack of suppression of fat signal or undesired suppression of water signal. Inhomogeneity is also a problem in spectroscopic imaging in the frontal and temporal lobes of the brain where broad magnetic shifts in the static field, B.sub.0, can cause fat or water to enter the passband of a spectrally selective excitation intended to excite, for instance, only NAA, or cause NAA signals to fall outside of the spectral passband.
The use of k-space trajectories for selective excitation in three spatial dimensions is heretofore known. See Pauly, U.S. Pat. No. 5,270,653, for example. The present invention is directed to using three-dimensional k-space trajectories for a spectral-spatial excitation where two dimensions are spatial and the third is the spectral or chemical shift dimension. Specifically, the center frequency of a spectral passband is varied as a function of two spatial coordinates.